Embodiments of the invention relate generally to a switching device for switching off a current in a current path, and more particularly to micro-electromechanical system based switching devices.
To protect against fire and equipment damage, electrical equipment and wiring must be protected from conditions that result in current levels above their ratings. Over-current conditions are classified by the time required before damage occurs and are grouped into two categories: timed over-currents and instantaneous over-currents.
Timed over-current faults are the less severe variety and require the protective equipment to deactivate the circuit after a given time period, which depends on the level of the fault. Timed over-current faults are typically current levels just above rated and up to 8-10 times rated. The system cabling and equipment can handle these faults for a period of time but the protective equipment should deactivate the circuit if the current levels don't recede. Typically timed faults result from either mechanically overloaded equipment or high impedance paths between opposite polarity lines—line to line, line to ground, or line to neutral.
Instantaneous over-currents, also termed short circuit faults, are severe faults and involve current levels of 8-10 time rated current and above. These faults result from low impedance paths between opposite polarity lines—line to line, line to ground, or line to neutral—and need to be removed from the system immediately. Short circuit faults involve extreme currents and can be extremely damaging to equipment and dangerous to personnel. The longer these faults persist on the system the more energy is released and the more damage occurs, it is of vital importance to minimize the response time and thus the let-through energy during a short circuit fault.
A circuit breaker is an electrical device designed to protect electrical equipment from damage caused by faults in the circuit. Traditionally, most conventional circuit breakers include bulky electromechanical switches. Unfortunately, these conventional circuit breakers are large in size thereby necessitating use of a large force to activate the switching mechanism. Additionally, the switches of these circuit breakers generally operate at relatively slow speeds. Further, these circuit breakers are disadvantageously complex to build, and thus expensive to fabricate. In addition, when contacts of a switching mechanism within a conventional circuit breaker are physically separated, an arc is typically formed between the contacts and continues to carry current until the current in the circuit ceases. Moreover, energy associated with the arc is generally undesirable to both equipment and personnel.
A contactor is an electrical device that is designed to switch an electrical load ON and OFF upon command. Traditionally, electromechanical contactors are employed in control gear, where the electromechanical contactors are capable of handling switching currents up to their interrupting capacity. Electromechanical contactors may also find application in power systems for switching currents. However, fault currents in power systems are typically greater than the interrupting capacity of the electromechanical contactors. Accordingly, to employ electromechanical contactors in power system applications it may be desirable to protect the contactor from damage by backing it up with a series device that is sufficiently last acting to interrupt fault currents prior to the contactor opening at all values of current above the interrupting capacity of the contactor.
Electrical systems presently use either a fuse or a circuit breaker to perform over-current protection. Fuses rely on heating effects (i.e., I2t) to operate. They are designed as weak points in the circuit and each successive fuse closer to the load must be rated for smaller & smaller currents. In a short circuit condition all upstream fuses see the same heating energy and the weakest one, by design the closest to the fault, will be the first to operate. Fuses however are one-time devices and must be replaced after a fault occurs.
Previously conceived solutions to facilitate use of contactors in power systems have include vacuum contactors, vacuum interrupters and air break contactors. Unfortunately, contactors such as vacuum contactors do not lend themselves to easy visual inspection as the contactor tips are encapsulated in a sealed, evacuated enclosure. Further, while the vacuum contactors are well suited for handling the switching of large motors, transformers and capacitors, they are known to cause damaging transient over voltages, particularly when the load is switched off.
Further, electromechanical contactors generally use mechanical switches. However, as these mechanical switches tend to switch at a relatively slow speed predictive techniques are required in order to estimate occurrence of a zero crossing, often tens of milliseconds before the switching event is to occur. Such zero crossing prediction is prone to error as many transients may occur in this time.
As an alternative to slow mechanical and electromechanical switches, fast solid-state switches have been employed in high speed switching applications. As will be appreciated, these solid-state switches switch between a conducting state and a non-conducting state through controlled application of a voltage or bias. For example, by reverse biasing a solid-state switch, the switch may be transitioned into a non-conducting state. However, since solid-state switches do not create a physical gap between contacts when they are switched into a non-conducing state, they experience leakage current. Further, due to internal resistances, when solid-state switches operate in a conducting state, they experience a voltage drop. Both the voltage drop and leakage current contribute to the generation of excess heat under normal operating circumstances, which may be detrimental to switch performance and life. Moreover, due at least in part to the inherent leakage current associated with solid-state switches, their use in circuit breaker applications is not possible.